Qualified volunteer supervisor here (National Trust and others). At that age it would really be a case of the parent(s) or guardian volunteering and bringing the children with them. NT have a minimum age of 18 below which youngsters must be supervised, and seeing as it is not always possible to arrange a staff supervisor the parent or guardian must be present. Other organisations have different limits, but they are nearly all bound by their insurance, risk assessments DBS (replacement for CRB) checks etc. So find something you want to do and take the children.